The nuclear matrix of the mammalian nucleus has been isolated and characterized. This cellular structural component appears to be involved in nuclear functions such as DNA synthesis. The nuclear matrix of the immature rat uterus has high affinity, saturable estrogen binding sites which appears to account for a considerable portion of the salt resistant nuclear estrogen receptors reported by others. The nuclear matrix of the uterus and prostate will be characterized and studied for their binding properties with sex steroid and antisteroid compounds. The nuclear matrix may be associated with a residual structural component extending throughout the cell and is termed the cell matrix. Steroid binding to these components will also be examined. A series of antiandrogens are being evaluated in experiments to elucidate the mechanism of action. Attention will be given to effects of these antiandrogens on DNA synthesis, nuclear uptake of androgens, testicular steroidgenesis, and binding to cellular receptors.